In the field of fiber optic transmission systems, it is sometimes desirable to control in a uniform manner the attenuation of a light beam output from an optical fiber for application in such fields as endoscopy, boroscopy and the like where controlled illumination of a viewing area is critical. (For purposes of clarification, the term "light beam" is used throughout this document to denote a collection of not only paraxial or generally parallel light rays, but also non-parallel light rays such as those which form a cone of light emitted from optical fibers or modified by optical devices). Several different methods for attenuating the intensity of such an output light beam have been utilized in the past in a variety of fields. One of the fast methods utilized for controlling the intensity of light emitted by an optical transmission system was the use of a high intensity lamp in which the amount of light generated therefrom was controlled by regulating the power supplied to the lamp. However, this method posed several problems such as changing the color of the generated light and unsuccessful operation of the lamp at lower intensities.
Other methods for attenuating the intensity of an output light beam involve the use of masking devices ranging from screens to masking plates having "image zones" with a plurality of minute apertures or slots formed in the mask's surface within each image zone (see FIG. 1a and 1b) to scissor-like instruments having angularly adjustable shielding members adapted for insertion into the path of the light beam (see FIGS. 2a and 2b). However, such masking type attenuators are not suitable for applications which require controllable and uniform illumination of a viewing or working field. In the use of such masking devices, significant variations in light intensity across the cross-sectional area of the output light beam exist due to the occurrence of diffraction, selective excitation of modes in fibers and fiber bundles, and the subsequent mode mixing (or lack thereof) inside these fibers and fiber bundles. In addition, the attenuation of the light beam over its divergence angle is non-uniform, and will cause both ring patterns and changes in NA.
A common cause of non-uniformity is caused by Fresnel diffraction. Fresnel rings are generated by virtue of the fact that every point on the primary, spherical wave front acts as a continuous emitter of spherical secondary wavelets (as shown in FIG. 3a). As the secondary wavelets propagate from the surface of the spherical wave front incident upon the aperture, their positive and negative wave components superimpose to form a diffraction pattern consisting of a plurality of alternating bright and dark rings concentric about a central bright spot (see FIG. 3b). The number of rings formed in the diffraction pattern depends upon the size of the aperture through which the wave front passes and the separation distance between the aperture and the observation plate (or object of an optical system), while the light intensity of the (bright) rings decreases proportionately with their increasing radial displacement from the central bright spot. The occurrence of Fresnel rings produces extreme variations in the intensity of light across the cross-sectional area of the output light beam, thereby greatly diminishing the illuminating capability of the optical transmission system. Due to the other non-uniformity factors and imperfections, such extremes in intensity variations are rarely observed. It was found, however, that these rings could be eliminated through the use of Fresnel zone plates which comprise a series of opaque, concentric rings positioned in the path of the light beam so as to correspond to either the bright or dark rings of the resulting Fresnel diffraction pattern. In this manner, the Fresnel zone plate prevents the superposition of the negative and positive wave components in adjacent zones and produces an single, point spot of high irradiance. Nonetheless, such a zone plate cannot be adapted for attenuation of the output light beam since this would require varying the surface area of the opaque rings which cannot be done without destroying its ring-eliminating capability.
Accordingly, masking-type attenuators which comprise a plate having a plurality of randomly or uniformly disposed holes or slots have been used in an attempt to provide a means of attenuating the output light beam while minimizing the effects of the ring patterns (see FIG. 1a and 1b). However, these attenuators do not produce uniform illumination since they still permit the formation of at least the fast order bright ring (the ring of highest intensity) around the centrally disposed bright spot. This results from the fact that the use of discrete holes (or slots) in the masking plate causes a hole (or slot) or lack thereof to be positioned at the center of the light beam. This in turn establishes a discontinuity (in this case, at the center of the light beam) in the amount of attenuation per unit radius over the radial extent of a perpendicular cross-section of the light beam. (As will be further discussed below, this amount of attenuation should continuously increase with respect to increasing length of the radius for complete elimination of the rings). In a particular embodiment of the slotted-type attenuator, as shown in U.S. Pat. No. 5,006,965, a plurality of slots are formed in an arcuate portion of a masking device or vane which is inserted perpendicular to the optical axis of the light beam. Although this device provides a dynamic range in the attenuation of the output beam, it does not completely eliminate the ring patterns due to the above-described discontinuity caused by the presence of the rectangular slots which are not evenly disposed radially with respect to the optical axis.
As shown in FIGS. 2a and 2b, the scissor-like attenuators provide variable adjustment of the radiant energy reflected off their extended shielding members across the path of the light beam to create uniform brightness of an image being viewed. Attenuation of the output light beam over the cross-sectional area of the beam occurs so as to radiate the different surfaces of the object with more or less radiant energy. In the first case shown in FIG. 2a, a peripheral portion of the light beam can be attenuated independently of its central portion (but not vice versa) so as to illuminate the central portion of the viewing field primarily. In the second case shown in FIG. 2b, which device is the subject of U.S. Pat. No. 4,706,657, the attenuator comprises two pairs of extended shielding members which operate either independently of or in cooperation with each other so as to provide for adjustable variation in the illumination of the viewing field at either or both its central and peripheral regions. As is apparent from the construction of the shielding members and the manner in which they are inserted into the path of the light beam, a plurality of rings patterns are likely to result again due to the discontinuity in the required radially increasing amount of attenuation over the radial extent of the light beam's cross-sectional area.
Another type of attenuating device consists of an optical lens having its surface treated either to mask out potions of the light beam or diffuse the light rays of the light beam. In the first case, the surface of the lens is coated with an attenuating substance such as a nickel or chrome layer which is deposited with a variable thickness depending upon the amount of attenuation desired. In the second case, electron scattering of ions is utilized to form a myriad number of minute grooves or holes which act to diffuse the light rays passing therethrough. In each instance, however, the treatment method is very costly due to the techniques used and the uniformity that must be achieved. Additionally, each lens produces only one level of attenuation, therefore requiring a large number of treated lenses to enable variable adjustment of the light beam's intensity.
Accordingly, it is an object of the present invention to provide a relatively inexpensive and simple attenuator capable of varying, either discretely or gradually, the light intensity of a light beam output from an optical transmission system.
It is another object of the present invention to provide an attenuator which attenuates the light intensity of a light beam by providing at least one masking member having a geometric shape and disposition with respect to a cross-sectional image spot of the light beam which extends from a center point to a circumference of the spot image and provides an increasing amount of attenuation over the radial extent of the image spot's area. It is a further object of the present invention to provide a discretely variable attenuator having a disc-like masking plate with a plurality of apertures formed in an arcuate control area at a periphery of the masking plate, the apertures having an origin disposed on a movement axis of the masking plate coincident with the optical axis of the light beam and a varying number of masking members per aperture so as to permit selection of a particular aperture corresponding to a particular degree of attenuation.
It is yet another object of the present invention to provide a gradually variable attenuator comprising a plurality of masking members each having two edges intersecting at a corner point of the member coincident with the optical axis of the light beam, the masking members being disposed along the optical axis at predetermined intervals and being rotatable independent of one another to permit variable adjustment of the total masking area over the area of the cross-sectional spot image.